Downhole tools use various types of sensors to test a downhole formation, analyze fluids, and perform other operations. Because the downhole environment has high temperatures, high pressures, harsh chemicals, and mechanical vibrations, the downhole tools must be mechanically designed to handle problems associated with such harsh conditions, and the downhole sensors must be able to operate with analytical accuracy and reliability. Added to these challenges, the downhole sensors must fit in the limited space available in the downhole environment, must be light weight and power efficient, and have a large dynamic range.
In the art, spectrophotometers, spectrometers, spectrofluorometers, refractive index analyzers, and similar devices have been used to analyze downhole fluids by measuring the fluid's spectral response. Each of these device typically use some form of EM radiation to perform its function (i.e., to analyze the fluid). In general, the wavelengths of the EM radiation can be in the x-ray, gamma, ultraviolet, visible, infrared or any combination of these ranges. When the radiation is detected, the response can identify characteristics of the analyzed fluid, such as the type of fluid (e.g., oil, water, and/or gas), the level of filtrate contamination, the hydrocarbon composition (e.g., amount of methane (C1), ethane (C2), propane (C3), etc.), the gas-to-oil ratio (GOR), etc. Knowledge of these characteristics can then be employed to model the reservoir, plan production, and perform other tasks.
A number of optical devices have been developed in the art for spectral analysis. For example, a spectrometer disclosed in U.S. Pat. No. 6,075,595 is based on light emitting diodes (LEDs) and is capable of imaging into a single optical channel. See also Malinen et al., “LED-based NIR Spectrometer Module for Hand-Held and Process Analyser Applications,” Sensors & Actuators B, vol. 51, no. (1-3), pp. 220-226 (1998). In another example, a multi-wavelength photometer uses seven LEDs, 1-mm plastic optic fibers, a 7×2 coupler, and two photodiodes. See Hauser et al., “A Multi-wavelength Photometer Based on Light-Emitting Diodes,” Talanta, vol. 42, no. 4, pp. 605-612 (1995). Still other small spectroscopes also use LEDs. See Cantrell et al., “The SLIM Spectrometer,” Analytical Chemistry, vol. 75, no. 1, pp. 27-35 (2003); See also Yeh et al., “Low Cost LED Based Spectrometer,” Journal of the Chinese Chemical Society, vol. 53, pp. 1067-1072 (2006). None of these devices is suitable for use in a downhole environment due to the harsh temperature and pressure requirements in the borehole environment.
Other devices disclosed in the art can be used downhole. In U.S. Pat. No. 6,476,384 to Mullins et al., for example, a device has a broadband halogen lamp source and has a mechanical chopper wheel driven by a motor. The lamp is imaged into an optical fiber bundle, and light from the bundle is directed to a photodiode used to synchronize the chopper wheel's motor. A calibration wheel driven by a rotary solenoid switch selects whether light from the bundle passes into a first path, a second path, or both. In the first path, light is directed to a light distributor forming part of a detector. In the second path, light is provided as input to a measurement cell and is afterward directed to the light distributor for the detector. The light distributor distributes the light received from the paths to a number of different channels with each channel having a lens, a bandpass filter, and a photodiode. While this device's broadband source does provide a number of spectral channels, the device must use a mechanical chopper, cannot perform synchronous detection, and requires a complex spectral detection system consisting of multiple photodiodes (i.e., one per spectral channel).
In U.S. Pat. Nos. 7,336,356 and 7,379,180 to Vannuffelen et al., a device has a broadband source. The device uses a rotating chopper wheel rotated by a motor to modulate the frequency of reference and measurement paths independently. For example, the measurement path has a first frequency and is split into two parts, and the reference path has a second frequency and is split into two parts. Each of these parts is then routed to multiple detection systems.
In US Pat. Pub. No. 2007/0109537, Vannuffelen et al. discloses an alternative approach that utilizes mechanical choppers and motors. Unfortunately, this approach, by design, is apparently limited to conventional raster scanning (CRS) spectroscopy, which involves scanning a plurality of sources or measurement wavelengths in a sequential fashion using a fixed time per channel (i.e. source or wavelength). As a consequence, CRS prevents synchronous detection of all spectral channels. Moreover, the device requires reference and measurement signals to be de-convolved using a single detector. Because the signal convolution using a single mechanical chopper results in shared harmonics, the device uses dual mechanical chopper assemblies to circumvent the complication of shared harmonics. Although this may simplify signal de-convolution, it adds further complexity to the devices and raises concerns relative to space, mechanical reliability, and accuracy.
Another device for downhole analysis of fluids disclosed in US Pat. Pub. No. 2007/0013911 to DiFoggio et al. provides Wavelength Modulation Spectroscopy (WMS). The device uses a narrow light beam source and a tunable optical filter (TOF). In additional disclosures of U.S. Pat. Nos. 7,280,214 and 7,362,422, both electrically tunable filters and mechanically (i.e. rotating) tunable filters are used for WMS. As purported, WMS eliminates the need for a second spectral reference channel. However, the devices have limited spectral range, which limits their use for downhole analysis of fluids. Specifically, each filter, whether electrical or mechanical in nature, possesses a limited tunable spectral bandwidth. To increase spectral range, the device requires multiple narrow band sources and tunable filters, which are mechanically cumbersome for the downhole environment. In addition, the device uses a single channel detection system that prohibits synchronous detection because the tunable optical filters are actuated using a single motor assembly, which gives each spectral channel a common fundamental frequency.
As disclosed in US Pat. Pub. No. 2008/0165356 to DiFoggio et al., another device has a laser diode array source containing a plurality of semiconductor light sources that enable conventional raster scanning (CRS) and Hadamard and synchronous Fast-Fourier Transform (FFT) scanning. However, the device lacks a way to dynamically scale the spectral response, and the device's sources lack a way for imaging a large number of spectral channels into a single spectral analyzer.
Therefore, in light of the above, what is lacking in the art is a viable broadband multi-channel source for downhole spectral analysis that enables self referencing, low-power operation, synchronous detection, and S/N improvement using discreet modulation of individual spectral channels.